[Django]-Django ModelForm: What is save(commit=False) used for?

Here it is the answer (from docs):

# Create a form instance with POST data.
>>> f = AuthorForm(request.POST)

# Create, but don't save the new author instance.
>>> new_author = f.save(commit=False)

The most common situation is to get the instance from form but only ‘in memory’, not in database. Before save it you want to make some changes:

# Modify the author in some way.
>>> new_author.some_field = 'some_value'

# Save the new instance.
>>> new_author.save()

From the Django docs:

This save() method accepts an optional commit keyword argument,
which accepts either True or False. If you call save() with commit=False,
then it will return an object that hasn’t yet been saved to the database.

In this case, it’s up to you to call save() on the resulting model instance.
This is useful if you want to do custom processing on the object before saving it,
or if you want to use one of the specialized model saving options.
commit is True by default.

It seems that save(commit=False) does create a model instance, which it returns to you. Which is neat for some post processing before actually saving it!

As a “real example”, consider a user model where the email address and the username are always the same, and then you could overwrite your ModelForm’s save method like:

class UserForm(forms.ModelForm):
    ...
    def save(self):
        # Sets username to email before saving
        user = super(UserForm, self).save(commit=False)
        user.username = user.email
        user.save()
        return user

If you didn’t use commit=False to set the username to the email address, you’d either have to modify the user model’s save method, or save the user object twice (which duplicates an expensive database operation.)

The basic thing here I understand is that it changes from a ‘form’ instance to a particular ‘model’ instance in view.

Let suppose I want to post an answer like this in StackOverflow.
The code would be like this:

# Create a form instance with POST data.
>>> form_data = AnswerForm(request.POST)

# Create, but don't save the new answer instance.
>>> Answer = form_data.save(commit=False)

So now we have to add the owner of this answer and save this in our database in the view page like this:

>>> Answer.owner = request.user

>>> Answer.save()

So like this, we can add the owner of this answer which we cannot do like form_data.owner = request.user in the view page and also not in the form class.

So basically, it changes from ‘form’ instance to ‘model’ instance and then lets you to modify the data and save it.

            form = AddAttachmentForm(request.POST, request.FILES)
            if form.is_valid():
                attachment = form.save(commit=False)
                attachment.user = student
                attachment.attacher = self.request.user
                attachment.date_attached = timezone.now()
                attachment.competency = competency
                attachment.filename = request.FILES['attachment'].name
                if attachment.filename.lower().endswith(('.png','jpg','jpeg','.ai','.bmp','.gif','.ico','.psd','.svg','.tiff','.tif')):
                    attachment.file_type = "image"
                if attachment.filename.lower().endswith(('.mp4','.mov','.3g2','.avi','.flv','.h264','.m4v','.mpg','.mpeg','.wmv')):
                    attachment.file_type = "video"
                if attachment.filename.lower().endswith(('.aif','.cda','.mid','.midi','.mp3','.mpa','.ogg','.wav','.wma','.wpl')):
                    attachment.file_type = "audio"
                if attachment.filename.lower().endswith(('.csv','.dif','.ods','.xls','.tsv','.dat','.db','.xml','.xlsx','.xlr')):
                    attachment.file_type = "spreasheet"
                if attachment.filename.lower().endswith(('.doc','.pdf','.rtf','.txt')):
                    attachment.file_type = "text"
                attachment.save()

here is my example of using save(commit=False). I wanted to check what type of file a user uploaded before saving it to the database. I also wanted to get the date it was attached since that field was not in the form.

In simple words, here we update the form object and let them know that don’t save the values in the database right now, we might change some input with instance and then use .save() to save all values in the database.

This gives us the flexibility to get all values from the HTML form and customize them according to our requirement and then save the instance.

• save(commit=False) cannot save objects but can return objects in a list and is used to customize the submitted inline objects in Django Admin. *As far as I researched and experimented, using save(commit=False) is the easiest way to customize the submitted inline objects in Django Admin.

• save() equivalent to save(commit=True) can save objects, then can retrun objects in a list and is not used to customize the submitted inline objects in Django Admin because more queries are run.

For example, there are Category model and Product model which has the foreign key of Category model as shown below:

# "my_app/models.py"

class Category(models.Model):
    name = models.CharField(max_length=20)

    def __str__(self):
        return self.name

class Product(models.Model):
    category = models.ForeignKey(Category, on_delete=models.CASCADE)
    name = models.CharField(max_length=20)

    def __str__(self):
        return self.name

And, there is overridden save_formset() with print(formset.save()) in Category admin which has Product inline as shown below. *save_formset() is run when changing inline objects:

# "my_app/admin.py"

class ProductInline(admin.TabularInline):
    model = Product
    extra = 0
    
@admin.register(Category)
class CategoryAdmin(admin.ModelAdmin):
    inlines = (ProductInline,)

    def save_formset(self, request, form, formset, change):
        print(formset.save()) # Here

And, there is Fruits category which has Apple and Orange products as shown below:

Now, I change Apple to Apple Juice and Orange to Orange Juice by clicking on SAVE as shown below:

Then, Apple is changed to Apple Juice and Orange is changed to Orange Juice as shown below:

Then, 2 inline objects are returned in a list on console as shown below:

[<Product: Apple Juice>, <Product: Orange Juice>]

And, two inline objects are saved by two UPDATE queries according to the PostgreSQL logs below. *You can see my answer explaining about how to log queries in PostgreSQL:

Next, I use formset.save(commit=False) as shown below:

# "my_app/admin.py"

# ...

@admin.register(Category)
class CategoryAdmin(admin.ModelAdmin):
    inlines = (ProductInline,)

    def save_formset(self, request, form, formset, change):
        print(formset.save(commit=False)) # Here

Now again, I change Apple to Apple Juice and Orange to Orange Juice by clicking on SAVE as shown below:

Then, Apple is not changed to Apple Juice and Orange is not changed to Orange Juice as shown below:

Then, 2 inline objects are returned in a list on console as shown below:

[<Product: Apple Juice>, <Product: Orange Juice>]

Then, two inline objects are not saved by two UPDATE queries according to the PostgreSQL logs below:

Next, I make the submitted names of inline objects uppercase with formset.save(commit=False) as shown below. *Again, save(commit=False) is used to customize the submitted inline objects in Django Admin:

# "my_app/admin.py"

# ...

@admin.register(Category)
class CategoryAdmin(admin.ModelAdmin):
    inlines = (ProductInline,)

    def save_formset(self, request, form, formset, change):
        for object in formset.save(commit=False): # Here
            object.name = object.name.upper()
        formset.save()

Now, I add Vegetable category, Cucumber and Potato products by clicking on SAVE as shown below:

Then, Cucumber and Potato are made uppercase as shown below:

And, two inline objects are saved by two INSERT queries according to the PostgreSQL logs below:

Next, I make the submitted names of inline objects uppercase with formset.save() as shown below. *Again, save() equivalent to save(commit=True) is not used to customize the submitted inline objects in Django Admin because more queries are run:

# "my_app/admin.py"

# ...

@admin.register(Category)
class CategoryAdmin(admin.ModelAdmin):
    inlines = (ProductInline,)

    def save_formset(self, request, form, formset, change):
        for object in formset.save(): # Here
            object.name = object.name.upper()
        formset.save()

Now again, I add Vegetable category, Cucumber and Potato products by clicking on SAVE as shown below:

Then again, Cucumber and Potato are made uppercase as shown below:

And first, two inline objects with Cucumber and Potato are saved by two INSERT, and second, two inline objects with CUCUMBER and POTATO are saved by two UPDATE queries according to the PostgreSQL logs below:

So, don’t use save() to customize the submitted inline objects in Django Admin because more queries are run. Instead, you should use save(commit=False) to customize the submitted inline objects in Django Admin.